Normal-weight women in the study who said they exercised vigorously five or more hours a week were 42% less likely to get pregnant in any given month than women who did not exercise at all.

The more vigorous the exercise that normal-weight women engaged in, the lower their probability for conception.

“We were surprised to find that even relatively small amounts of vigorous activity seemed to impact fertility,” says researcher Lauren A. Wise, ScD, an associate professor of epidemiology at the Boston University School of Public Health.

Moderate Exercise Benefits Everyone

The study included about 3,000 women who were trying to get pregnant and were not receiving infertility treatments.

Activity level was measured through a single questionnaire the women filled out after entering the study.

Running, aerobics, gymnastics, swimming, and intense bicycling were considered vigorous exercise, while brisk walking, leisurely cycling, golfing, and gardening were considered moderate exercise.

A major goal of the study was to determine if the impact of physical activity on time to pregnancy varied by body weight.

Among the findings:

Vigorous exercise did not appear to delay conception times in women who were overweight or obese.

Engaging in five or more hours of vigorous exercise a week reduced the likelihood of getting pregnant in a given month by almost half, among women who had tried unsuccessfully to conceive for several months before entering the study.

Moderate exercise for any length of time was associated with small decreases in time to pregnancy for women of all body types.

“The take-home message for overweight and obese women is that any exercise seems better than none,” Wise tells WebMD. “Being overweight is a risk factor for infertility, and these findings suggest that exercise may improve fertility in these women.”

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More Study Needed, Researchers Say

Wise says the reliance on a single, self-reported questionnaire to determine each participant’s exercise intensity was a major limitation of the study.

It is also not clear if specific types of vigorous exercise impact fertility more than others, she adds.

“We can’t really tell a woman who swims every day to cut back, because we don’t know if that would be beneficial,” she says. “We need additional studies to be able to say that one type exercise impacts fertility but another doesn’t.”

Texas ob-gyn Robert Brzyski, MD, PHD, says women who do not exercise at all should be encouraged to do so when they are trying to conceive.

He adds that most women who are already active should be able to continue to exercise without worrying that it will negatively impact their fertility.

Brzyski is a professor of obstetrics and gynecology at the University of Texas Health Science Center in San Antonio.

“Women who exercise to relieve stress should certainly not stop when they want to become pregnant unless there is a medical reason for doing so,” Brzyski says. “It may be a good idea to cut back some, but physical activity should be encouraged.”